Chapter 7: Chemical Reactions and Stoichiometry

Empirical Formulas and Combustion Analysis

Empirical formulas represent the simplest whole-number ratio of elements in a compound. Combustion analysis is a common laboratory technique used to determine the empirical formula of organic compounds.

• Empirical Formula: The simplest integer ratio of atoms in a compound.

• Combustion Analysis: Involves burning a sample in excess oxygen and measuring the amounts of CO2 and H2O produced to determine the amounts of C and H present.

• Calculation Steps:

  1. Convert grams of CO2 and H2O to moles of C and H.

  2. If O is present, subtract the mass of C and H from the total mass to find O.

  3. Divide by the smallest number of moles to get the simplest ratio.

• Example: A compound yields 2.20 g CO2 and 0.90 g H2O. Calculate moles of C and H, then determine the empirical formula.

Grams, Moles, and Molecular Mass

Converting between grams and moles is fundamental in stoichiometry. The molecular mass (or formula weight) is used as a conversion factor.

• Molecular Mass (M): The sum of atomic masses of all atoms in a molecule (in g/mol).

• Conversions:

  • From grams to moles:

  • From moles to grams:

• Example: Convert 10.0 g of NaCl to moles. , so

Balancing Chemical Equations

Balanced chemical equations ensure the law of conservation of mass is obeyed. Each side of the equation must have the same number of atoms of each element.

• Steps:

  1. Write the unbalanced equation.

  2. Balance atoms of elements that appear only once on each side first.

  3. Balance hydrogen and oxygen atoms last.

  4. Check your work.

• Example:

Stoichiometry and Mole Ratios

Stoichiometry involves using balanced equations to relate quantities of reactants and products.

• Mole Ratio: The ratio of coefficients from the balanced equation.

• Conversions: Use mole ratios to convert between moles of different substances.

• Example: In , 2 moles of H2 produce 2 moles of H2O.

Limiting Reagent and Excess Reagent

The limiting reagent is the reactant that is completely consumed first, limiting the amount of product formed. The excess reagent is the reactant that remains after the reaction is complete.

• Determining the Limiting Reagent:

  1. Calculate moles of each reactant.

  2. Use stoichiometry to determine which reactant produces the least amount of product.

• Calculating Excess Reagent Remaining: Subtract the amount consumed from the initial amount.

• Example: If 5.0 g of A reacts with 10.0 g of B, determine which is limiting and how much of the excess remains.

Theoretical Yield and Percent Yield

Theoretical yield is the maximum amount of product that can be formed from given reactants. Percent yield compares the actual yield to the theoretical yield.

• Theoretical Yield: Calculated using stoichiometry from the limiting reagent.

• Percent Yield:

• Example: If the theoretical yield is 8.0 g and the actual yield is 6.0 g, percent yield is

Combustion Reactions

Combustion reactions involve a substance reacting with oxygen to produce oxides and energy, often as heat and light.

• General Form: (for hydrocarbons)

• Example:

Electrolytes and Nonelectrolytes

Substances that dissolve in water to produce ions are called electrolytes. Those that do not produce ions are nonelectrolytes.

• Ionic Substances: Usually strong electrolytes; dissociate completely in water.

• Molecular Substances: May be nonelectrolytes or weak electrolytes; do not dissociate completely.

• Example: NaCl is a strong electrolyte; sugar (C6H12O6) is a nonelectrolyte.

Dissociation in Aqueous Solution

When ionic compounds dissolve in water, they separate into their constituent ions.

• Example:

• Prediction: Use solubility rules to predict if a compound will dissociate.

Acids and Bases

Acids are proton (hydrogen ion) donors, while bases are hydroxide ion donors.

• Acids: Donate H+ ions in solution (e.g., HCl).

• Bases: Donate OH- ions in solution (e.g., NaOH).

• Strong vs. Weak: Strong acids/bases dissociate completely; weak acids/bases dissociate partially.

• Example: HCl is a strong acid; acetic acid (CH3COOH) is a weak acid.

Molecular, Total Ionic, and Net Ionic Equations

Chemical reactions in aqueous solution can be represented in three ways:

• Molecular Equation: Shows all reactants and products as compounds.

• Total Ionic Equation: Shows all strong electrolytes as ions.

• Net Ionic Equation: Shows only the species that actually change during the reaction.

• Example:

  • Molecular:

  • Total Ionic:

  • Net Ionic:

Predicting Products and Precipitation

Predicting the products of a reaction involves understanding the types of chemical reactions and solubility rules.

• Precipitate Formation: Occurs when an insoluble product forms in solution.

• Example: Mixing solutions of Na2SO4 and BaCl2 forms insoluble BaSO4 as a precipitate.

Limiting Reagents and Reaction Completion

Sometimes, reactions are limited by the amount of one or more reagents, which determines the maximum amount of product formed.

• Key Point: The reaction stops when the limiting reagent is consumed.

Oxidation and Reduction (Redox Reactions)

Redox reactions involve the transfer of electrons between species. Oxidation is the loss of electrons, and reduction is the gain of electrons.

• Oxidation: Loss of electrons; increase in oxidation number.

• Reduction: Gain of electrons; decrease in oxidation number.

• Assigning Oxidation Numbers: Rules are used to assign oxidation states to each atom in a compound or ion.

• Stoichiometric Calculations: Use balanced redox equations to calculate quantities of reactants and products.

• Example: In , Na is oxidized (0 to +1), Cl is reduced (0 to -1).

Table: Comparison of Strong and Weak Acids/Bases

Additional info: These notes expand on the learning objectives by providing definitions, examples, and equations for each key concept in stoichiometry and chemical reactions, suitable for exam preparation.